The invention relates to audible frequency tone generation and more particularly to generation of different audible frequency tones which are played for announcing someone at a door, for annunciating the time of day, for paging employees at a department store, and so forth. Specifically, the invention is directed to generation of audible frequency tones having good tonal quality from digitally encoded information and to a musical door chime preferably also combined with a clock for both announcing the presence of someone at a door and for annunciating the time of day.
There is a substantial amount of prior art in the field of electronic audible frequency tone generation, especially in the area of electronic organs. Sophisticated electronic circuits are provided in some electronic organs for not only playing individual musical notes in response to actuation of individual keys in the keyboard, but also for generating chords comprising a plurality of musical notes or even a rhythm pattern in response to actuation of an additional key or keys. Generally speaking, however, electronic organ audible frequency tone generating circuits are too expensive to be considered for a commercially feasible musical door chime.
Recently, however, less sophisticated electronic audible frequency tone generating circuits have been proposed for musical door chimes. U.S. Pat. Nos. 3,878,750 and 4,043,240, for example, disclose electronic audible frequency tone generating circuits for playing a musical tune when a means, such as a door pushbutton, is actuated. The musical door chimes disclosed by these patents include a memory for storing digitally encoded representations of each note of a musical tune and hard-wired digital circuitry connected to the memory for addressing and decoding each musical note for generating a squarewave having the frequency and the duration of the note. The generated squarewave energizes a loudspeaker for playing the musical note.
There are several disadvantages to the musical door chimes disclosed by U.S. Pat. Nos. 3,878,750 and 4,043,240. In the first place, in order to generate the frequencies of some musical notes with accuracy, an additional frequency correction circuit must be included which adds to the complexity of the hard-wired digital circuitry and to the cost. In the second place, even if a frequency correction circuit is included, as shown in U.S. Pat. No. 4,043,240, a squarewave is used for energizing a loudspeaker which results in poor tonal quality.